Spinal Cord Injury Basics

The spinal cord connects the base of the brain to the body’s nerves, making it an important communication highway between the brain and the rest of the body. It is surrounded and protected by a bony canal, which is part of the spinal column (also known as the spine or backbone). A spinal cord injury happens when the spinal column fractures or when the ligaments holding the spinal column together fall out of alignment. As a result, the spinal cord within the bony canal may be bruised or crushed. This injury interrupts the communication between the brain and the parts of the body below the site or level of injury.

Anatomy and Physiology

The delicate nerve tissue of the spinal cord is protected by the spinal column (also known as the spine or backbone).

Bony Structure

• Vertebrae can be crushed by force pushing on it and making a fracture; it is the same mechanism that happens when someone breaks an arm or a leg. Vertebrae can also be damaged when the ligaments that hold them together break; this makes the stacked vertebrae fall out of alignment. Both falling out of alignment and breaks can injure the spinal cord. Injuries can range from the spinal cord being bruised or, in a worst case scenario, being crushed and interrupted.
• More than two dozen bones stacked upon one another form the spine. The one closest to the head is called the atlas because it carries the weight of the skull. Individual bones are grouped together. From top to bottom, there are seven neck bones (C, for cervical), 12 chest-level bones (T, for thoracic), 5 lower back bones (L, for lumbar), 4 fused bones forming the sacrum (S), and the coccyx or tailbone.
• Each bony unit is called a vertebra; vertebrae is the plural form of the word. A vertebra is made up of the body and the arch. The arches align and form the spinal canal when the vertebrae are stacked.
• There are “cushions” between each vertebra; these intervertebral disks are made up of cartilage, collagen fibers, and water. Ligaments hold the vertebrae and the intervertebral disks together.

Spinal Cord

• The spinal cord is composed of many nerve fibers that run from the base of the brain to the small of the back. It is the most important way for the brain to communicate with the rest of the body. The spinal cord runs within the spinal canal.
• Spinal nerves exit from the spinal cord to send and receive signals from muscles, skin, and other organs. The nerves exit the bony canal through passageways called foramen between the vertebrae. Nerve exiting near the neck and upper back send and receive signals from the arms. Nerves that exit in the chest area are in charge of internal organs, and nerves that exit in the lower back control the legs and the genital area.
• The spinal cord and the spinal canal are of equal lengths before birth. The spinal cord stops growing earlier than the spinal canal that covers it. In an adult, the spinal cord ends in the spinal canal in the small of the back, around the level of the second lumbar vertebrae (L2). The rest of the spinal canal only contains spinal nerves and not the spinal cord itself.

What is a level of injury

• The level of injury refers to the point where the spinal cord is injured; it marks a border between areas of the body which are affected and not affected by the spinal cord injury.
• Spinal cord levels of injury are named according to the corresponding vertebra and spinal nerve exiting from the spinal canal in that area. For example, you may hear your physicians talk about a C6 level. This means the spinal cord is injured at the level of the backbone’s 6th cervical (neck) vertebra; muscles and other organs at the level of C6 and above still have nerves connected to them while those below C6 have lost communication with the spinal cord and brain.
• Likewise, a T9 level corresponds to injury at the 9th thoracic (chest-level) vertebra and an L2 level corresponds to the 2nd lumbar (lower back) vertebra.

Abilities after injury

Patients’ abilities after they are injured depend on the level of injury. A spinal cord injury that happens at T2 or lower would leave use in the arms, but would affect the connection between the brain and the legs. This is known as having paraplegia. An injury that happens in the cervical or neck area – C1 to C7 and T1 – would impact both arm and leg function. This is known as tetraplegia, which is another word for quadriplegia.

Patients who have an injury above the C4 level – C1, C2, and C3 – will need a mechanical ventilator to help them breathe. The C4 level of the spinal cord provides the motor signals that control the diaphragm, which is a muscle under the lungs. The diaphragm is the main force that makes the lungs expand when a person breathes in. The diaphragm does not work well enough when there is a C4 level or above injury for patients to breathe on their own.

The extent of the injury also changes from person to person. It depends on which parts of the spinal cord are injured and how badly the cord was injured. Some people can move their arms, but cannot feel light touch or pinpricks. Other people have sensation, but no movement. Others may have no sensation and no movement.

Complete and Incomplete Injuries

There are complete and incomplete spinal cord injuries.

• Complete injury: the connections between the brain and the end of the spinal cord are interrupted. Recovery of sensation and movement is not very likely.
• Incomplete injury: there are still some nerve connections remaining between the brain and the end of the spinal cord. Chances for recovery are better than in a person with a complete injury.

ASIA Exam

An American Spinal Injury Association (ASIA) exam is a standard physical examination performed to help classify and try to predict recovery for a patient who has a spinal cord injury. This helps determine which parts of the body are working normally and which parts are affected by the spinal cord injury. The exam is extensive because it covers the whole body. A physician trained in performing the exam will test strength and sensation.

Motor Exam

The motor exam tests five key muscles in each arm and leg; there are 20 key muscles that are tested overall. Each key muscle corresponds to a level in the spinal cord. A C5 level corresponds to bending the elbow, for example, and an L3 level corresponds to straightening the knee.

Sensory Exam

There are two parts of the sensory exam. Light touch and pinprick are tested separately and each spot on the patient’s skin corresponds to a level in the spinal cord. For example, sensation on the tip of the patient’s middle finger corresponds to C7 and sensation behind the knee corresponds to S2. Light touch and pinprick are tested separately because they travel in different nerve pathways in the spinal cord.

The pinprick pathway travels closer to the motor pathway than the light touch pathway. Recovering sensation in the pinprick pathway can better predict recovery than light touch in the motor pathway. For example, if the patient has normal light touch sensation at the tip of the finger, the chance of getting strength back in the muscles which straighten the elbow would be about 1 in 10. If there were normal pinprick sensation at the tip of the finger, however, the odds of getting strength back to the elbow muscle group would be at least 6 in 10.

Anal Muscle

The very end of the spinal cord sends motor signals to the external anal sphincter, which is the muscle that contracts when the patient is trying to hold a bowel movement. The same nerves that control that muscle send light touch and pinprick signals back to the spinal cord. The physician will ask the patient whether he or she can feel pinprick and light touch in the anal area. The patient also is asked to squeeze the physician’s gloved finger when it is inserted in the rectum.

This is an important last step in the ASIA exam. If the motor and sensory functions are intact on the rectal exam, the injury is incomplete. If the patient cannot contract the anal muscle and cannot feel light touch and pinprick, the injury is considered complete.